Acoustic well logging techniques and tools are extensively described in the art. Acoustic well logging is used to provide surveys of formations traversed by earth boreholes. In particular, measurements are made of the velocities of acoustic waves to reveal valuable information concerning the type of rocks and the porosity of the rocks in the formation surrounding the borehole. A commonly measured acoustic parameter is the velocity of compressional waves. However, other acoustic wave parameters such as the velocity of shear waves and other modes of acoustic energy are also useful in evaluating the formation.
An acoustic well logging tool for measuring the velocity of acoustic waves typically employs a sonic pulse transmitter and a plurality of sonic receivers selectively spaced from the transmitter. The sonic receivers include transducers to convert the incident acoustic wave to an electrical waveform and suitable amplifiers to transmit the waveforms to surface located processing equipment.
The spacings between the sonic receivers and the transmitter influence the character of the parameter measurement. For example, a longer spacing enables a greater effect of the formation characteristics on the sonic wave with the possibility that wave analysis can thus reveal more useful data about the formation. On the other hand, a longer spacing results in the arrival of greatly attenuated sonic waves making their analysis more difficult. When many spaced sonic receivers are employed to measure the velocity of the acoustic wave, the need to maintain adequate spacings between each of these receivers results in strongly attenuated waves.
Current sonic well logging tools commonly use one transmitter with a pair of sonic receivers. A double pair of transmitter-receivers has been used in the art, for example to provide compensation for physical misalignments of the tool in the borehole as described in the U.S. Pat. No. 3,304,537 to Schwartz. A three receiver, single transmitter sonic well logging tool has been described in U.S. Pat. No. 3,390,377 to Elliott et al. Generally, when the number of receivers is increased to improve the velocity measurement, special techniques are needed to process the increase in the flow of signals and enhance the accuracy of sonic wave parameter measurements.
The use of computers in determining any of several correlation functions between two signals is a well established field. Much literature has been written on the subject and general reference may be made, for example, to a book entitled "Statistical Communication and Detection" written by E. A. Robinson and published by the Hafner Publishing Company of New York in 1967. Of particular interest in this book is the section dealing with numerical filtering methods for digital computers. Various correlation techniques are described.